A magnetic recording medium of the so-called coating type that a binder with ferrogmagnetic powder dispersed therein is coated on a non-magnetic support has heretofore been in widespread use. Recently, however, with the increasing requirements of high density recording, a metallic thin film type of magnetic recording medium with a ferromagnetic metallic thin film as a magnetic recording layer, said ferromagnetic metallic thin film being formed by a vapor deposition method, such as vacuum deposition, sputtering and ion plating, or a plating method, such as electric plating and non-electric plating, has received attention and has been put into practical use.
This metallic thin film type of magnetic recording medium is very advantageous in electromagnetic properties since a ferromagnetic metal of high saturation magnetization can be formed into a very thin layer without the use of a non-magnetic substance, such as a binder, and thus it is suitable for high density recording.
The metallic thin film type of magnetic recording medium, however, suffers from the serious problem that the metallic surface is corroded with the lapse of time after the production thereof, resulting in a reduction of electromagnetic properties.
Various methods have been proposed to solve the foregoing problem. One method is to provide a protective layer on the metallic surface by coating with thermoplastic or thermosetting polymers, as described in W. German Patent Application (OLS) Nos. 220964 and 322006, British Patent Application 2106011A and Japanese Patent Application (OPI) Nos. 50128/83 and 158037/82. (The term "OPI" as used herein means a "published unexamined Japanese patent application".) This method, however, fails to provide sufficient corrosion resistance since the thickness of the protective layer cannot be increased because of the spacing loss between a head and the magnetic layer (see FIG. 2). In addition, a method of nitriding the surface of the magnetic layer (see Japanese Patent Application (OPI) No. 33806/75) and a method of oxidizing the surface (see Japanese Patent Publication No. 20025/67) are known. However, the time required for these methods is as long as 10 minutes to 2 hours; if the time of treatment is shortened, no sufficient anticorrosion effect can be obtained.
Another serious problem of the metallic thin film type of magnetic recording medium is that running properties and durability are poor; the magnetic recording medium sticks to a running path during running, preventing smooth running or sometimes causing the running to stop, or the still durability is poor.
As a method to overcome the foregoing problem, it has been considered to coat the ferromagnetic metal thin film with thermoplastic or thermosetting polymers, or lubricants such as aliphatic acids and aliphatic acid esters, as described in Japanese Patent Application (OPI) Nos. 68930/81 and 80827/81. Even when this method is employed, the running properties are sometimes improved only insufficiently, or the durability is also sometimes improved insufficiently. In order to ensure sufficiently satisfactory running properties and durability, it is inevitably necessary to increase the thickness of the coating layer. This will lead to the spacing loss and finally to deterioration of electromagnetic properties. It is also necessary to increase the thickness of the coating layer so that sufficiently satisfactory corrosion protection can be obtained. This is also not preferred for electromagnetic properties.